According to the product pages for the https://store.google.com/product/nexus_6p and 5X: https://store.google.com/product/nexus_5x, both device conform to the USB-C standard instead of QuickCharge 2.0.6P:
On one side, it's great because it's a future-proof standard instead of a proprietary technology.
On the other side, and only ship a 5V charger.
Because the connector is of USB Type-C instead of Micro-USB, they pump up to 3A instead of 2A (QuickCharge 1.0 / Samsung) or 2.4A (Apple)
But 15W is less than to 9V / 2A (18W) or 12V / 1.8A (21.6W) from current QuickCharge 2.0 devices specs.
3A on 5V also means any long cable will waste energy, substantially less than 5V making it to the device depending on the cable quality and length.
Maybe these two phones will also accept 12V input as well (unlikely 20V) as it's part of the USB power spec.
That'll be up to the reviewers to determine and measure that.
Meanwhile, don't expect these phones to charge as fast as the current Motorola or Samsung generation.
As mentioned in the presentation today, they'll still charge a lot faster than the iPhones.
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